I didn't build the site but from talking to the developer its flash and html based to allow features like SEO. There are also many advanced features in the Pro version and many more being worked on.

Its designed for web designers who don't want to have to code a site. There are a few designers working with circlepad creating business sites that can easily be managed by the user.

On the code side its built in such a way that adding features and updates are very easy and don't require the server to be restarted. This allows customers to request feature and sometimes see them added in as little as a week. As web designers it would be great to hear what you think is missing.